The present invention relates to an apparatus for controlling a pressure-activated actuator, and also to an apparatus for controlling the recirculation of engine exhaust gas.
A pressure-activated actuator has a pressure chamber and a movable partition provided in the pressure chamber and dividing the chamber into two chambers. The partition moves when the pressure within the pressure chamber changes. When the partition moves this way, the actuator drives a device coupled to it. Among the devices which can be driven by the pressure-activated actuator is an EGR (Exhaust-Gas Recirculation) control valve. The EGR control valve is usually actuated by a pressure-adjusting valve which in turn is controlled by a computer.
While a diesel engine is operating, air is always supplied to the engine in excess. The ratio of the supplied air to the amount of air required by the engine, i.e., the excess-air ratio, depends on the speed at which the diesel engine is rotating. The excess-air ratio is great in the low-load side of the engine. Therefore, the amount of NOx in the low-load side cannot be sufficiently reduced unless the exhaust-gas recirculation is controlled to a greater degree than in the high-load side of the diesen engine. In FIG. 7 attached hereto, curve A shows the relationship between the output and speed of the diesel engine, curve B represents the relationship between the road-load on the engine and the speed thereof, and region C (a shaded area) indicates the region in which the EGR control cannot be effective. The exhaust-gas recirculation is controlled such that air is supplied to the diesel engine with excess-air ratio .lambda. of 50% to 70% when the engine operates in idling region D or region E. In region E, the engine is driving wheels at an automobile speed of about 40 km/h. As is shown in FIG. 7, region F in which the excess-air ratio is .lambda..sub.2, which is less than ratio .lambda..sub.1, surrounds region E, and region G in which the excess-air ration is .lambda..sub.3, which is less than ratio .lambda..sub., surrounds region F. When the driver quickly and deeply depresses the accelerator pedal in order to raise the automobile speed from 40 km/h, the amount of injected fuel is increased and the diesel engine abruptly goes into region C, before the engine speed raises. Consequently, the exhaust-gas recirculation can no longer be controlled. In order to prevent the engine from belching black smoke, it is necessary to lower the EGR control. In other words, the exhaust-gas recirculation must be controlled whenever the operating condition of the engine changes
There are two methods of electronically controlling the exhaust-gas recirculation, i.e., feedback control and duty control. The feedback control uses an EGR valve equipped with a potentiometer. The potentiometer detects the distance the valve actually moves to control the exhaust-gas recirculation. If this distance is different from the distance for which the valve should move to achieve an optimum EGR control, the signal representing the difference is fed back to the valve, thereby to control the pressure within the EGR valve. The duty control consists in periodically opening the EGR valve to the atmosphere. That is, a duty control is performed to automatically change the pressure within the EGR valve to a desired valve.
The feedback control is disadvantageous in the following respects. First, the system for achieving this control is expensive. Secondly, the actuator used in this control, and/or the EGR valve driven by the actuator has an insufficient durability. The duty control has also some drawbacks. First, the response of the EGR valve is poor since the valve must be repeatedly opened to the atmosphere, and a pressure must be applied into the EGR valve. Secondly, the ERG valve causes ripples, inevitably reducing the durability of the valve and greatly changing the pressure within the valve. In order to minimize the changes in this pressure, the diameter of a throttle provided between the opening of the valve connecting the atmosphere and the pressure source provided for the EGR valve can be reduced. The measure, if taken, will lower the response of the EGR valve.